Centrifugal pumps of conventional design operate at very low efficiency when they are required to produce flow rates that are fractions of their maximum or design flow rate. Consequently, when centrifugal pumps are employed in systems requiring variable flow rates and operate in such systems at low flow rates, they waste considerable power.
The wasted power is dissipated, in largest part, as an increase in the temperature in the fluid being pumped; and in some instances, the resulting increase in temperature presents difficulties.
By way of example, one-may consider centrifugal fuel pumps used in aircraft. Comparatively high fuel flow rates are required to develop high power needed for take-offs, climbs to altitude, emergency situations or in the case of military aircraft such as fighters, for sudden bursts of speed or aerobatic flight maneuvers. At the same time, however, when the engines are operated in so-called flight idle descents, ground idle settings or taxiing, the fuel flows typically may be only about 1.5% to 3% of the flow rates required for high power settings. As a consequence, the wasted power may cause the fuel being pumped to overheat. It may then boil, or decompose. Sludges, gum, cokes or vapors in fuel lines, fuel control valves and injection nozzles may result; and each may interfere with fuel flow, engine power control and system reliability.
The problem can be compounded in aircraft where fuel being pumped to the engine is heated by other means. For example: at extremely high aircraft speeds, impact of ambient air on heat exchangers causes excessive drag and local temperatures which are so high that these cannot be used for some cooling requirements. Consequently, while rejection of such heat to the fuel to be burned is desired, it often cannot be practiced to the extent desirable because of excessive heating of the fuel from power wasted in the fuel pumps at low flow rates. Other problems arise also from contaminants in the fluids, such as dust, sand, fibrous particles or corrosive substances, which cause pump parts to plug up, stick, erode or wear excessively and thus cause the pump to become unreliable or inoperative.
The present invention is directed to overcoming one or more of the above problems.